With a recent demand for low fuel consumption to automobiles, there have been required raw rubber materials such as conjugated diene-based (co)polymer rubbers from which rubber compositions for tires having low rolling resistance, excellent in wear resistance and failure characteristics, and high in wet skid resistance which is a representative index of driving stability can be prepared.
What is necessary to reduce the rolling resistance of tires is just to lower the hysteresis loss of vulcanized rubbers. The hysteresis loss can be evaluated using various properties as indexes. For example, raw rubber materials having high impact resilience at 50 to 80° C., a low tan δ at 50 to 80° C. or low Goodrich heat build-up are preferred. The raw rubber materials with low hysteresis loss include natural rubber, isoprene rubber, butadiene rubber and the like. However, these have the problem of low wet skid resistance.
On the other hand, in recent years, there have been proposed methods of using an inorganic filler such as silica as a reinforcing agent, or an inorganic filler and carbon black in combination. Tire treads using the inorganic filler or both the inorganic filler and carbon black are low in rolling resistance and excellent in driving stability represented by wet skid resistance. However, they have the problem of poor wearing resistance, tensile strength and the like of vulcanized rubbers. Part of the reason for this has been considered to be the fact that the affinity of inorganic filler for the conjugated diene-based rubber is lower than that of carbon black, so that a sufficient reinforcing effect can not be obtained.
In view of the above-mentioned problems, the present inventors have proposed a rubber-inorganic compound composite material containing (I) a conjugated diolefin (co)polymer rubber having a primary amino group and an alkoxysilyl group bonded to a polymer chain and (II) an inorganic compound (JP-A-2004-067982). Then, the conjugated diolefin (co)polymer rubber (I) used herein has the primary amino group and the alkoxysilyl group on the polymer chain, and the weight average molecular weight of the rubber is usually form 100,000 to 2,000,000. Accordingly, the alkoxysilyl group of the above-mentioned conjugated diolefin (co)polymer rubber reacts with silica which acts as a filler, and the amino group reacts with carbon black which acts as a filler, resulting in the formation of the rubber-inorganic compound composite material having affinity for these fillers. A rubber composition using this composite material is useful as tires or vibration insulators. However, the conjugated diolefin (co)polymer rubber (I) of JP-A-2004-067982 has a weight average molecular weight as high as 100,000 to 2,000,000, similarly to normal rubber. Accordingly, when these fillers are blended, the alkoxysilyl group or amino group existing in the molecular chain reacts with silica or carbon black to increase the melt viscosity in kneading, which causes poor processability. As a result, an increase in product percent defective and a rise in production cost are liable to cause problems.